1. Field of the Invention
The invention relates to a regulating device.
2. Description of the Prior Art
In a burner the ratio of the amount of air to the amount of fuel, referred to as the air ratio or lambda, must be matched to each other throughout the entire output range either by a control arrangement or by a regulating arrangement. In general terms lambda should be slightly above the stoichiometric value of 1, for example 1.3.
Air ratio-regulated burners, unlike controlled burners, react to external influences which alter combustion. For example, combustion can be re-regulated after a change in the kind of fuel or the density of the air. They have a higher level of efficiency and thus a higher degree of effectiveness as well as lower levels of pollutant and soot emissions. Environmental pollution is less and the service life is increased.
Regulation of the air ratio is particularly effective if the quality of combustion can be observed with a sensor. Typically, known burners use oxygen sensors in the flue gas duct, temperature sensors on the burner surface or UV-sensors in the combustion chamber. More recent developments are based on the ionisation electrode which has already long been used as standard for monitoring the flame in burners.
Air ratio-regulated burners which use an ionisation electrode as the flame sensor are known from German patent specification No 196 18 573. Such burners monitor the regulating circuit inter alia by a procedure whereby the measurement signal should not depart from a safety margin around the reference regulating value during the regulating operation, for a long period. If however that is the case, the burner shuts down.
It is at least less meaningful for the air ratio to be regulated immediately after the burner fires up as the ionisation signal is only representative of combustion in the thermally steady-state condition. Therefore the ratio of air and fuel is initially controlled, for example during the first minute after the burner comes into operation. It is only thereafter that it is subjected to precise correctional regulation.
It is further known that, during the firing operation, the air ratio is varied so that it is possible to find a mixture which is good for the kind of fuel supplied. In the further starting operation the burner is controlled on the basis of that air ratio value. An example thereof is also described in German patent specification No 196 18 573. During the firing operation such a burner raises the gas proportion with a fixed volume flow of air until the ionisation electrode detects a flame. The start-up control retains the gas valve position corresponding to firing, although the gas-air mixture is typically somewhat too rich. It is only after the system has reached its operating temperature that it is switched over to regulation by means of an ionisation signal.
Besides the burner starting characteristics, it is conceivable that at a later time, for other reasons, the ionisation signal is not representative of combustion or the regulating circuit becomes unstable due to external influences. In that case also the regulation can be temporarily shut down and the air ratio can be controlled during that time.
The control period should be as short as possible as external influences cannot be the subject of correctional regulation during that time. In addition, the quality of the control action under the specific circumstances involved should be monitored at least marginally and for plausibility. If the position of the fuel valve or the air blower is not monitored by additional measures during the control period, then in the event of a defect the permissible emission values can be severely exceeded.
The object of the present invention is to improve quality monitoring during such control periods in an inexpensive and simple fashion.
In accordance with the present invention, there is provided a regulating device for an air ratio-regulated burner, which burner is equipped with
a sensor which detects the quality of combustion, and
a setting member which influences the feed amount of fuel or the feed amount of air in dependence on a setting signal,
which regulating device is equipped with
a sensor evaluating device which is connectable downstream of the sensor and which produces a sensor signal,
a control unit in which characteristic data for determining at least one mode of behaviour of the setting member are stored and which at least at times produces at least one control signal, and
a regulator which produces the setting signal during at least one control period in dependence on the control signal and not in dependence on the sensor signal and otherwise in dependence on the sensor signal, wherein
at least at times during the control period the regulator produces a comparison signal in dependence on the sensor signal,
the regulating device establishes the difference between the comparison signal and a corresponding signal, and
the regulating device can produce a fault signal in dependence on the difference.
Advantageous aspects of the invention are set forth in the dependent claims.